1. Field of the Invention
This invention relates to a method and to an apparatus for detecting whether the getter material has been correctly evaporated during manufacture of a cathode-ray tube and especially a picture tube for color television.
2. Description of the Prior Art
A cathode-ray tube (CRT) is formed by an evaporated glass envelope having a front portion known as a faceplate which forms a screen. The faceplate is joined by means of a funnel-shaped portion to the rear cylindrical portion known as a neck, the electron gun or guns being housed at the end of said neck.
The final stages of manufacture of the picture tube are as follows: a cup containing the getter material to be evaporated is attached to the electron gun which is installed within the envelope. There then follow the successive steps of evacuation of the envelope by pumping, evaporation of the getter material by means of a heating induction coil, heating of the cathode or cathodes to a temperature which is higher than the usual operating temperature in order to form and stabilize the cathode material. Voltages are then applied to the grids of the electron gun with a view to cleaning the tube or in other words removing the undesirable gas particles which are absorbed by the getter material. If this material has not evaporated, the picture tube thus fabricated will not be capable of operating correctly by reason of the poor quality of vacuum but primarily by reason of the presence of positive ions which will be attracted by the cathode and cause damage to this latter.
It has been found that, in mass-production lines, evaporation of getter material does not take place correctly in about 1% of cathode-ray tubes being manufactured. Tubes which exhibit this defect then have to be reprocessed or in other words withdrawn from the production line and returned to the getter evaporation station. This incident may have a number of different causes: error of operator, faulty positioning of the induction heating coil employed for the evaporation process, faulty coil or current supply source, and so on.
Up to the present time, evaporation of getter material has been checked by visual observation of the tube walls. This type of checking operation is not wholly reliable since the operator may fail to detect a transient evaporation fault as a result of inattention. It should be added that visual observation is a tedious operation.
The invention described below provides a remedy for the drawbacks outlined in the foregoing.